Efficient Partial Snapshot Implementations
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In this work, we propose the λ-scanner snapshot, a variation of the snapshot object, which supports any fixed amount of 0 < λ≤ n different SCAN operations being active at any given time. Whenever λ is equal to the number of processes n in the system, the λ-scanner object implements a multi-scanner object, while in case that λ is equal to 1, the λ-scanner object implements a single-scanner object. We present the λ-Snap snapshot object, a wait-free λ-scanner snapshot implementation that has a step complexity of O(λ) for UPDATE operations and O(λ m) for SCAN operations. The space complexity of λ-Snap is O(λ m). λ-Snap provides a trade-off between the step/space complexity and the maximum number of SCAN operations that the system can afford to be active on any given point in time. The low space complexity that our implementations provide makes them more appealing in real system applications. Moreover, we provide a slightly modified version of the λ-Snap implementation, which is called partial λ-Snap, that is able to support dynamic partial scan operations. In such an object, processes can execute modified SCAN operations called PARTIAL_SCAN that could obtain a part of the snapshot object avoiding to read the whole set of components. In this work, we first provide a simple single-scanner version of λ-Snap, which is called 1-Snap. We provide 1-Snap just for presentation purposes, since it is simpler than λ-Snap. The UPDATE in 1-Snap has a step complexity of O(1), while the SCAN has a step complexity of O(m). This implementation uses O(m) CAS registers.
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